Spermatogonial stem cell (SSC) transfer is a valuable methodology for generations of genetically modified animals, and has been proposed as a method for preserving genetic traits of commercially valuable animals. Fertility declines in stud bulls after about five years of age, and ongoing researches have been conducted to develop technology for transferring SSC from one bull to another.
In SSC transfer, testis cells from a donor bull testis are obtained either by castration or biopsy. The donor testis cells can be enriched for type Ad spermatogonium through a variety of methods including flow sorting for cell surface markers and growth in culture media. The enriched donor cells are then cultured under conditions allowing expansion of SSCs. Expanded SSCs are injected into the rete testis of a recipient for transplantation. Depending on the animal breed, functional donor spermatozoa cells can be produced in the recipient three to five months after the transplantation.
Currently, a key barrier for SSC transfer is that sperm produced from the recipient can include a mixture of sperm derived from the donor as well as native sperm produced by the recipient. After SSC transfer, the recipient animal continues to produce its own spermatozoa; as a result, even with the best existing SSC transfer practices, after transplantation, less than 50% of the spermatozoa produced by the recipient animal are derived from the donor animal, and more typical percentages of spermatozoa derived from the donor animal are in the range of 1% to 20%.
Attempts have been made to increase the yield of spermatozoa derived from the donor animal after SSC transfer. Existing methods include the use of radiation or chemicals (such as busulfan) to kill native (recipient) stem cells; however, these methods only achieved suboptimal effects. Accordingly, improved methods for SSC transfer are needed.